1. Field of the Invention
The present invention relate to an oxide thin film transistor (TFT) and its fabrication method, and more particularly, to an oxide TFT having a bottom gate structure using amorphous zinc oxide-based semiconductor as an active layer, and its fabrication method.
2. Description of the Related Art
As the consumer's interest in information displays is growing and the demand for portable (mobile) information devices is increasing, research and commercialization of light and thin flat panel displays (“FPD”), which substitute cathode ray tubes (CRTs), the conventional display devices, has increased. Among FPDs, the liquid crystal display (“LCD”) is a device for displaying images by using optical anisotropy of liquid crystal. LCD devices exhibit excellent resolution, color display and picture quality, so they are commonly used for notebook computers or desktop monitors, and the like.
The LCD includes a color filter substrate, an array substrate and a liquid crystal layer formed between the color filter substrate and the array substrate.
An active matrix (AM) driving method commonly used for the LCD is a method in which liquid crystal molecules in a pixel part are driven by using amorphous silicon thin film transistors (a-Si TFTs) as switching elements.
The structure of a general LCD will now be described in detail with reference to FIG. 1.
FIG. 1 is an exploded perspective view showing a related art LCD device.
As shown in FIG. 1, the LCD includes a color filter substrate 5, an array substrate 10 and a liquid crystal layer 30 formed between the color filter substrate 5 and the array substrate 10.
The color filter substrate 5 includes a color filter (C) including a plurality of sub-color filters 7 that implement red, green and blue colors, a black matrix 6 for dividing the sub-color filters 7 and blocking light transmission through the liquid crystal layer 30, and a transparent common electrode 8 for applying voltage to the liquid crystal layer 30.
The array substrate 10 includes gate lines 16 and data lines 17 which are arranged vertically and horizontally to define a plurality of pixel areas (P), TFTs (T), switching elements, formed at respective crossings of the gate lines 16 and the data lines 17, and pixel electrodes 18 formed on the pixel areas (P).
The color filter substrate 5 and the array substrate 10 are attached in a facing manner by a sealant (not shown) formed at an edge of an image display region to form a liquid crystal panel, and the attachment of the color filter substrates 5 and the array substrate 10 is made by an attachment key formed on the color filter substrate 5 or the array substrate 10.
The LCD as described above is light and has low power consumption, as such, the LCD receives much attention, but the LCD is a light receiving device, not a light emission device, having a technical limitation in brightness, a contrast ratio, a viewing angle, and the like. Thus, a new display device that can overcome such shortcomings is being developed.
An organic light emitting diode (OLED), one of new flat panel display devices, is self-emissive, having a good viewing angle and contrast ratio compared with the LCD, and because it does not require a backlight, it can be formed to be lighter and thinner. Also, the OLED is advantageous in terms of power consumption. Besides, the OLED can be driven with a low DC voltage and has a fast response speed, and in particular, the OLED is advantageous in terms of a fabrication cost.
Recently, research for an increase of a size of an OLED display device is actively ongoing, and in order to achieve such a large-scale OLED display device, development of a transistor that can secure constant current characteristics as a driving transistor of an OLED to ensure a stable operation and durability is required.
An amorphous silicon thin film transistor (TFT) used for the above-described LCD may be fabricated in a low temperature process, but has a very small mobility and fails to satisfy a constant current bias condition. Meanwhile, a polycrystalline silicon TFT has a high mobility and satisfying constant current bias condition but fails to secure uniform characteristics, making it difficult to have a large area and requiring a high temperature process.
Thus, an oxide semiconductor TFT including an active layer formed with oxide semiconductor, but in this case, if oxide semiconductor is applied to an existing TFT of a bottom gate structure, the oxide semiconductor is damaged during an etching process of source and drain electrodes, causing degeneration.
FIG. 2 is a sectional view sequentially showing the structure of a related art oxide TFT.
As shown in FIG. 2, the oxide TFT of a bottom gate structure is configured such that a gate electrode 21 and a gate insulating layer 15 are formed on a substrate 10, and an active layer 24 made of oxide semiconductor is formed on the gate insulating layer 15.
Thereafter, source and drain electrodes 22 and 23 are formed on the active layer 24. In this case, when the source and drain electrodes 22 and 23 are deposited and etched, the lower active layer 24 (in particular, a portion ‘A’) is damaged to cause degeneration, degrading the reliability of the device.
Namely, metal for the source and drain electrodes is limited to molybdenum-based metal in consideration of a contact resistance with the oxide semiconductor. In this case, however, when the source and drain electrodes are formed through a wet etching, the active layer is damaged due to the physical properties of the oxide semiconductor which is vulnerable to an etchant, and even when the source and drain electrodes are formed through a dry etching, the active layer is degenerated due to back-sputtering and oxygen deficiency of the oxide semiconductor.